In the Third Age, the Lhûn formed the border between Mithlond and Arnor.[2] During the War with Angmar, it formed the end of the Witch-king's influence: many of the Dúnedain fled across it. When Eärnil II came, passage was won back over it. With their defeat imminent, many of the Witch-king's minions drowned in the river Lhûn.[3]

The river Lhûn found its origin in the First Age or before, but following the War of Wrath, its course was severely altered. Its original course is no longer recorded in history, but after the breaking of the Blue Mountains, it flowed in the Gulf of Lune.[4] The river had its origin in the north of the Blue Mountains, and had two tributaries: the Little Lune and an unnamed river that had its origin in the Emyn Uial.

The meaning of Lhûn is not known. In connection with its first appearance, in a manuscript dating from ca. 1940, the translation "Blue River" is given.[5][6] Apparently, Tolkien originally envisioned it as Noldorin for "blue" (cf. Sindarinluin).[7]Lhûn, and especially the initial /lh/, may have been valid in Noldorin, it was not so in Sindarin, so Tolkien had to rewrite the etymology. He considered the following:[6][8]